In the industrial work place, toxic substances are often found in the air. The permissible level of exposure is set by government regulations, and compliance therewith requires that the breathing air be monitored. This is best done when the workman wears a personal air sampler, which continuously sniffs the ambient air during the 8 hour long workday. Such a personal sampler may, for example, be worn in a shirt pocket and will slowly suck ambient air through a combined filter-indicator tube by means of a dry-cell powered electric pump.
An early example of such a personal air sampler is that of Garner, U.S. Pat. No. 3,410,059. This sampler tends to suck the ambient air fast at first, and then slower, as the filter-indicator tube gradually clogs up and the batteries run down. Thus, the indication is not linearly representative of each work hour, but over emphasizes conditions earlier in the day, when the work place might be less messed up and less dangerous.
The later developed personal air sampler of Sipin, U.S. Pat. No. 3,953,152, is an improvement, since, by use of a voltage control circuit and, by restricting the motor operating conditions to avoid high IR losses, the motor speed does not vary very much, over the course of the work day, from that selected.
The instant system is an improvement on the prior art in that the speed of the motor is controlled to be substantially constant over the course of the work day, despite slow clogging of the filter-indicator tube and despite running down of the batteries.
In order to maintain the speed of the motor constant at its selected value, the actual speed of the motor is measured by continuous monitoring of the armature voltage through a linear voltage following amplifier having a signal voltage output differentially combined with a reference voltage of the desired value to produce an error signal, which is used as a feedback signal to control the application of power to the motor. Furthermore, the actual speed is not measured by use of a separate tachometer, but by sensing the internal back EMF of the motor through the linear voltage following amplifier during the intervals (when it is coasting) between short pulses of full voltage to the motor terminals, the signal voltage output of the amplifier being cut off for the duration of such pulses. The use of short pulses at full voltage permits successful operation of the system at very low motor speeds, where friction would prevent proper operation using steady d.c. of low terminal voltages.
The system described herein has two aspects. On the one hand, an improved personal air sampler is achieved. On the other hand, the herein disclosed new electric motor control circuit, generally useful in other applications, is used in the personal air sampler.